Cats and dogs have a digitigrade stance. That is that they perpetually walk on their toes, as opposed to their toe nails like a deer or flat-footed like a bear. This puts their heel off the ground. People often mistake the heel for a backwards facing knee but a close examination of the anatomy shows that they have a knee that points much the same way as ours. The knee is easy to miss when looking at the animal because the knee is about at the same level as the belly. The knees of some digitigrade animals such as dogs or cheetahs are easier to see because they are distinctly below the level of the belly. On the front limb, the wrist is low on the leg and slightly angled forward. Most of the limb that you see below the belly is a long forearm (radius and ulna).
In addition to the six muscles you have seen in the bear and the deer, there are two more ones that make prominent bulges on the bodies of cats. Cats use their forearms dynamically and have more developed back and shoulder (latissimus dorsi and deltoid) muscles than you see in other quadrupeds.
Print out the mammal anatomy worksheet and follow the In the step by step guide below, adding one muscle at a time. Then envision how these muscles show through skin and fur. This will help you memorize the shapes and locations of the muscles better than just reading this post. Click on the first image to start a sideshow with step by step details.
The mountain lion has a digitigrade stance walking on the pads of the toes and the ball of the foot. Note that the heel and wrist form the joints that are held off the ground but still low on the leg.
The gastrocnemius or calf muscle originates at the back of the femur and inserts into the tip of the heel bone. The space between the tendon near the heel and the leg bone makes a prominent divot in the back of the leg and can be seen in many mammals.
The crural triceps originate on the pelvis and femur and insert into the top of the knee cap. This is the equivalent of the quadriceps in the human. This bulky muscle makes a prominent bump at the front of the thigh.
The biceps femoris is an exception to the generality that muscles become smaller as they move away from the core of the body. This muscle originates from the end of the pelvis and fans out in a big triangle to the base of the knee and across the gastrocnemius. The rear edge of this muscle makes a ridge along the back of the leg in short haired animals.
The latissimus dorsi is a broad muscle that originates along the spine and rib cage and inserts into the forearm (humerus). This muscle makes a distinct ridge on the side of the body of the lion.
The triceps brachialis is the large muscle at the back of the upper arm. It originates along the base of the shoulder blade and the humerus and inserts into the tip of the ulna at the elbow.
The extensor carpi radialis is a ropey muscle in the front of the forearm. It originates near the base of the humerus and inserts into the wrist. This muscle gives some thickness to the forearm but not as much as you see in the upper arm.
The deltoids or shoulder muscles are thick and strong in cats. They originate along the spine of the shoulder blade (scapula) and insert near the top of the forearm (humerus).
The brachiocephalicus is a thick muscle that sits on either side of the neck and turns the head side to side. The lower edge of this muscle often makes a prominent grove called the jugular grove. The muscle originates at the back of the skull and inserts into the upper arm (humerus).